The present invention relates to a display memory for storing pixel data to be supplied to pixels of a display, a driver circuit having a display memory and driving pixels arrayed in a matrix of the display by signals corresponding to the image data, a display using the driver circuit, and a portable information device.
Liquid crystal displays are being widely used as display systems of mobile phones and PDAs (Personal Digital Assistants) and other portable information devices by making use of their light weight, thinness, low power consumption, and other features. Further, due to the spread of mobile phones and the Internet, the displays of portable information devices are being required to be further enlarged in size, offer color, and otherwise be improved in quality and are being strongly required to be ultra-low in power consumption for realizing long hours of usage. In liquid crystal drivers, therefore, it has become important to realize lower power consumption while handling larger screens and color.
In conventional liquid crystal drivers, the power consumption of the logic circuits inside the LSI has been lowered by a variety of methods, but if dealing with the enlargement of size of screens or colorization and other improvements in image quality, the number of drive devices increases, so the power consumption rises accordingly.
In order to realize lower power consumption, the method of building a display memory (also referred to as a “frame memory”) into a liquid crystal driver has been employed. This eliminates the need for a controller memory for transfer of display data, slashes the number of parts, and realizes a reduction of the power consumption.
Further, a new drive system may be employed to reduce the power consumption.
Concerning this subject, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-64514 discloses a liquid crystal driver having a built-in general purpose memory realizing high speed and lower power and a liquid crystal display using that driver.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-293144 discloses a liquid crystal display device using a liquid crystal driver with a built-in memory generating graphics with a low power consumption and at a high speed and able to reduce the load of the CPU.
Further, Japanese Unexamined Patent Publication (Kokai) No. 7-281634 discloses a liquid crystal display using a liquid crystal driver with a built-in memory achieving lower power consumption and realizing high speed graphic drawing access.
Further, Japanese Unexamined Patent Publication (Kokai) No. 7-230265 realizes a liquid crystal drive device improving the means of supply of power and having a built-in memory with a low power consumption and a large capacity.
Further, Japanese Unexamined Patent Publication (Kokai) No. 7-175445 discloses a technique achieving lower power consumption and higher speed graphic drawing without lowering the operating efficiency of the system by building into the liquid crystal driver a display memory accessible by a general purpose memory interface.
In the layout of an LSI of a liquid crystal driver having a built-in conventional display memory, however, the interface has terminals at one side of the general purpose memory cells, so general purpose interface signal interconnects must be detoured around them. Power is taken for the amount of those interconnects.
Further, a conventional display memory uses data buses, address buses, and control signal buses for display and graphics drawing and requires bus arbitration. Due to this, if the number of accesses for display is large, the time for the drawing is reduced.
Further, in the conventional system, the memory is accessed from the CPU for every group of pixels. Therefore, for example, when desiring to store one screen's worth of data from the CPU into the memory, (one screens's worth of number of pixels)/(number of pixels in group of pixels) of write operations to the memory are required, so the number of times of operation of the memory was large. The operating power consumption of the memory is proportional to the number of times of read/write operations, therefore consequently an increase of the power consumption is induced.
Further, when transferring display data from the memory to the liquid crystal panel, one horizontal line on the display screen's worth of the display data was simultaneously output, but the data was read from the memory for this purpose not in amounts of one horizontal line at one time, but in amounts of an output data line of the liquid crystal driver.
For example, when desiring to display one screen's worth of data stored in a memory on an LCD display screen, (one screen's worth of number of pixels)/(group of pixels) of read operations of the memory become necessary, so there is the disadvantage that power of the amount of the number of times of access is consumed.
Further, in the conventional system, the operation has to be performed at the high frequency of the memory. No margin can be given to the access time of the CPU. Therefore, there is a disadvantage that this is not suited display of a moving picture requiring quick switching of the screen.
Further, when using a conventional memory, the images of the memory array and the pixel array of the liquid crystal are not the same, so it is necessary to calculate where a pixel is in the memory at the time of drawing.
Further, a conventional display memory rewrites all data to be written at one time when writing data. Accordingly, when there is a data which is not desired to be changed in the data to be written at one time, a so-called read-modify-write system which reads out the data in advance before rewriting the data, modifies the bits to be rewritten while masking the data not desired to be rewritten, and then writes the data into the memory is employed. For this reason, there were the disadvantages that the number of operation was large and power was consumed.
Further, conventionally, when outputting image data stored in a display memory to a digital/analog converter (DAC), since RGB data corresponding to the three primary colors of the color cannot be output in a time division manner, the outputs of the display memory were directly connected to DACs in one-to-one correspondence. In this way, conventionally, since a DAC was necessary for every RGB data, the number of DACs was large and an increase of the power consumption was induced.
In order to reduce the power consumption of the DACs, the settling time must be adjusted. Since the operating speeds of the DACs and the display memory are different, they must be separately controlled. Depending on the characteristics of the DACs, the phases of the input signals must be adjusted. Conventionally, however, when outputting the data of the display memory to the DACs, the timing for outputting the RGB data is fixed. The phase of the data cannot be freely changed to match with the characteristics of the DACs, so this necessity could not be coped with.
Further, in order to lower the power consumption of a liquid crystal display, there is the method of lowering the power supply voltage. When the operating power supply voltage becomes smaller than 3.0V, however, malfunctions occur. Further, for a method of supplying power considering power conservation, there is a partial display mode used in a standby screen of mobile phones, but in this partial display mode, although nothing is displayed on the screen, leakage current of the memory cells still flows, so there is the disadvantage of consumption of power.